The present invention relates to a silver(II) oxide cell and its manufacture. More particularly, it relates to improvements in silver(II) oxide cells comprising silver(II) oxide as the positive active material and an alkaline solution as the electrolyte.
Advantageously, the silver(II) oxide cell provided by this invention has a large discharge capacity and shows flat and even discharge characteristics without any substantial deterioration in performances on storage. Further, it has a low internal resistance and, even in case of a high rate discharge, can show a high closed circuit voltage.
Attempts have heretofore been made to use silver(II) oxide as a positive active material for alkaline cells, since silver(II) oxide has a larger theoretical discharge capacity than various other materials as shown in the following table:
______________________________________ Positive active Theoretical discharge capacity material mAH/ml mAH/g ______________________________________ AgO 3222 433 Ag.sub.2 O 1650 231 MnO.sub.2 1901 378 HgO 2752 247 ______________________________________
However, silver(II) oxide shows the discharge voltages in two steps due to its discharge reaction as follows: Ag.sup.++ .fwdarw.Ag.sup.+ .fwdarw.Ag. For instance, in FIG. 1, of the accompanying drawings which shows the variation of the terminal voltage with the discharge time on the discharge of a cell of Type G12 (JIS (Japanese Industrial Standard )) or Size 43 (IEC (International Electrochemical Commission)) using silver(II) oxide itself as the positive active material with a loading resistance of 2 k.OMEGA., the discharge curve has a higher voltage part of about 1.8 V and a lower voltage part of about 1.5 V. Such unflatness in the discharge voltage is unfavorable for use as a positive active material for an electric source of constant voltage in electronic wrist watches, hearing aids, measuring instruments and the like.
In addition, silver(II) oxide is apt to be decomposed in an alkaline electrolyte, and a large deterioration of the discharge characteristics is seen on the storage of a cell comprising silver(II) oxide as the positive active material. The gas (O.sub.2) evolved on the decomposition of silver(II) oxide provides the increase of the internal pressure in the cell, which causes the leakage of the electrolyte and/or induces the deformation or rupture of the cell. Further, the evolved gas leads to the inactivation of the negative active material and/or the deterioration of the electrolyte or the separator.
In order to overcome the above drawbacks, there have been made some proposals including: (1) chemical or electro-chemical reduction of a molded product of particles of silver(II) oxide in an appropriate shape to make a thin layer of silver(I) oxide at the surface; (2) application of a binding agent comprising silver(I) oxide onto the surface of a molded product of particles of silver(II) oxide in an appropriate shape to form a coating layer comprising silver(I) oxide; (3) insertion of a molded product of particles of silver(II) oxide into a can wherein a metal (e.g. zinc, copper, nickel, silver) readily oxidizable with silver(II) oxide is plated on the inner surface or a screen made of the said metal is accommodated to contact closely to the inner surface, whereby silver(I) oxide is produced at the interface between the molded product and the metal plated layer or the screen; and (4) vapor-plating or spattering silver on the surface of a molded product of particles of silver(II) oxide to form a silver plated layer and oxidizing the silver plated layer with the silver(II) oxide to make a thin layer of silver(I) oxide, etc.
In method (1), however, the formed thin layer of silver(I) oxide has many micropores, and therefore the alkaline electrolyte penetrates through such micropores into the inner part of the molded product. Thus, the electrolyte contacts directly to silver(II) oxide, whereby the latter is decomposed to cause the depression of the discharge capacity.
In method (2), the application of a binding agent comprising silver(I) oxide onto the molded product usually produces a coating having pinholes or unevenness, and a satisfactory effect is hardly obtainable. Further, in case of the binding agent having functional groups such as hydroxyl or carboxyl, silver(II) oxide is reduced on contact with them so that the discharge capacity is decreased. Furthermore, on admixing silver(I) oxide with the binding agent or applying the resultant mixture onto the surface of the molded product, the silver(I) oxide is readily reacted with carbon dioxide in the air to give silver carbonate, which may result in the deterioration of the electrolyte or the inactivation of the negative active material.
In method (3), the surface of the molded product is uneven so that silver(1) oxide is produced at the convex portions which contact with the metal plated layer or the screen but not at the concave portions which do not contact with them. Thus, the entire molded product can not be completely covered by silver(I) oxide.
In method (4), the silver plated layer sometimes has pinholes, through which the electrolyte penetrates into the inner part of the molded product so as to cause the decomposition of silver(II) oxide. The formation of the pinholes can be prevented by making the silver plated layer thick but, in such case, the penetration of the electrolyte into the molded product to contact with the silver(I) oxide under the silver plated layer is prevented so that any discharge reaction will not proceed.
As the result of the extensive study, it has now been found that the use of a molded product of particles, each particle comprising a core part of silver(II) oxide and a surface part of silver(I) oxide integrally and continuously provided thereon, can overcome the said drawbacks as seen in the conventional methods. This invention is based on the above finding.